This invention relates to a ring oscillator comprising a plurality of cascaded inverting stages, each stage comprising an input circuit for detecting an output voltage of a preceding stage.
Such oscillators are widely known in the art of integrated circuit manufacture and may conveniently be constructed using simple inverting logic circuits (NOT, NAND, NOR and so on) as the inverting stages. The circuits may, for example, be TTL or CMDS logic circuits so that, as well as being very simple, the oscillator may be integrated conveniently with other circuits. The current output of each stage takes a finite time to charge or discharge the input capacitance of the following stage to the threshold voltage. The number of inverting stages is odd and the stages are cascaded in a loop so that at a certain frequency a 180-degree phase shift is imparted to signals passing around the loop. Provided the loop gain is large enough, the signals soon become non-linear and square-wave oscillations are produced which can be used for a variety of purposes. In metal-oxide-semiconductor (MOS) integrated circuits, ring oscillators are commonly used to drive `charge pump` circuits for providing a bias voltage, for example, for substrate biasing, as disclosed in U.S. Pat. No. 4,142,114, for example.
Despite being both self-starting and relatively simple, such ring oscillators suffer from large uncertainty in the frequency of oscillation, due to variations both in the process used to fabricate the circuit and in operating conditions such as supply voltage and temperature. This problem arises because the frequency is determined solely by parameters inherent to the inverter stages and the devices used to construct them for a given number of inverters.
Therefore, the known oscillators cannot easily be designed to have a well-defined oscillating frequency, and in particular the natural frequency of such oscillators tends to be higher than is desirable. Because the inverters are operating at a frequency close to their maximum operating frequency and because the circuitry driven by the oscillator is normally of the same logic family as the inverters, that circuitry too is operated at a frequency close to its limit and operation of the circuit as a whole may be degraded unnecessarily. It should be appreciated that the frequency cannot be reduced merely by increasing the number of stages indefinitely because other modes of oscillation (harmonics) can occur at higher frequencies, and in practice this limits the number of stages to about seven or nine.